1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for controlling pressure in a fluid delivery system. More specifically, embodiments of the invention relate to a method and apparatus for dampening pressure fluctuations in a process fluid delivery system used for substrate processing.
2. Description of the Related Art
The need for greater process control continues to increase as semiconductor and electronics processing industries continue to strive for larger production yields and smaller device sizes. As a result, better control over substrate processing parameters and more precise methods of process endpoint detection are desirable. Additionally, the desire to reduce production costs creates a demand for a reduction in the cost of the substrate processing equipment that may provide improved process control.
Substrate processing typically requires many deposition and etching steps in order to fabricate an electronic device. The deposition and etching steps are performed in one or more substrate processing chambers which contain process fluids that act on the substrate, and precise control of the process fluid pressure, such as a process gas pressure, for example, is often required to achieve the desired processing result. Additionally, some endpoint detection methods may also require precise control of the process fluid pressure to reliably detect a processing endpoint.
One example of such an endpoint detection method is optical emission spectrometry (OES) which may be used to detect the endpoint of a deep trench plasma etch process. The trench depth combined with a narrow line width may make conventional etch endpoint detection methods (e.g., laser interferometry) unreliable, and deep trench etching applications typically require prompt cessation of the etching process once the desired depth has been achieved to avoid damage to a device feature or substrate support. The OES detector includes an optical sensor which provides spectroscopic information on the plasma chemistry which may change as the etching proceeds through different material layers. An etch endpoint is reached when a pre-determined change in plasma chemistry and associated spectral change is detected by the OES detector. However, the change in plasma chemistry may be so small that fluctuations in the process gas pressure within the processing chamber may mask the endpoint or falsely trigger an endpoint. Depending upon the application, acceptable variations in gas pressure may be limited to a few tenths (or less) of one psia (pounds per square inch absolute) in order for the OES detector to operate reliably.
Efforts to precisely control process fluid pressure and minimize pressure fluctuations have resulted in process fluid delivery systems which utilize pressure regulators and pressure insensitive mass flow controllers. Pressure regulators are often used to reduce the effects of pressure variations that may occur in a fluid delivery system due to cross talk, for example, which can result from single line drops within the system. However, pressure regulators are primarily used to step pressure up or down and have limited capability to dampen small pressure fluctuations since the regulating action of the regulator can produce small pressure fluctuations.
Pressure insensitive mass flow controllers (PIMFCs) have been developed which incorporate a pressure sensor in addition to the thermal flow sensor and control valve of a conventional mass flow controller (MFC). The pressure sensor is placed upstream of the thermal flow sensor and is part of the control loop which operates the control valve so that the PIMFC can quickly adjust the control valve to compensate for any for any changes in the inlet pressure detected by the pressure sensor. As a result, a PIMFC may effectively dampen pressure fluctuations down to a fairly small scale (e.g., 1-2 psia), but smaller pressure perturbations of a few tenths of one psia may pass through the PIMFC unaffected which may adversely affect substrate processing or endpoint detection.
Therefore, a need exists for a cost effective pressure dampening method and apparatus which can effectively dampen small pressure perturbations in a fluid delivery system.